A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate can be measured accurately. These alignment sensors are effectively position measuring apparatuses. Different types of marks and different types of alignment sensors are known from different times and different manufacturers. A type of sensor widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Pat. No. 6,961,116 (den Boef et al). Generally marks are measured separately to obtain X- and Y-positions. However, combined X- and Y-measurement can be performed using the techniques described in published patent application US 2009/195768 A (Bijnen et al). The contents of both of these applications are incorporated herein by reference.
Advanced alignment techniques using a commercial alignment sensor are described by Jeroen Huijbregtse et al in “Overlay Performance with Advanced ATHENA™ Alignment Strategies”, Metrology, Inspection, and Process Control for Microlithography XVII, Daniel J. Herr, Editor, Proceedings of SPIE Vol. 5038 (2003). These strategies can be extended and applied commercially in sensors of the type described by US'116 and US'768, mentioned above. A feature of the commercial sensors is that they measure positions using several wavelengths (colors) and polarizations of radiation (light) on the same target grating or gratings. No single color is ideal for measuring in all situations, so the commercial system selects from a number of signals, which one provides the most reliable position information.
There is continually a need to provide more accurate position measurements, especially to control the overlay error as product features get smaller and smaller. One cause of error in alignment is sensitivity of the position sensor to sub-resolution features present in the mark. To explain, alignment marks are generally formed of gratings with features far larger than the features of the device pattern which is to be applied to the substrate in the lithographic apparatus. The required positioning accuracy is therefore obtained not by the fineness of the alignment grating, but rather by the fact that it provides a periodic signal that can be measured over many periods, to obtain overall a very accurate position measurement. On the other hand, a very coarse grating is not representative of the actual product features, and therefore its formation is subject to different processing effects than the real product features. Therefore it is customary for the coarse grating of the alignment mark to be made up of finer product-like features. These finer gratings are examples of the “sub-resolution” features referred to above, being too fine to be resolved by the alignment sensor. They may be referred to more commonly as “at-resolution” features, however, by reference to the resolving power of the patterning system in the lithographic apparatus. More discussion of these issues and different forms of sub-segmented marks are described in Megens et al, “Advances in Process Overlay—Alignment Solutions for Future Technology Nodes” in Metrology, Inspection, and Process Control for Microlithography XXI, Proc. of SPIE Vol. 6518, 65181Z, (2007), doi: 10.1117/12.712149.
The alignment marks are typically applied to the substrate at an early stage in a device manufacturing process, using a lithographic apparatus similar or even identical to the one which will apply the patterns for subsequent product layers. The at-resolution features become subject to slightly different errors in their positioning than the coarser alignment grating features, for example due to aberrations in an optical projection system used to apply the pattern. The effect of this in current alignment sensors is that the measured position contains unknown errors, being neither the position of the coarse grating nor that of the finer at-resolution grating. It has further been found that, depending on the color and polarization used in the position measurement, the error in the reported position caused by a mismatch in position between the coarse and fine gratings can be much greater than the mismatch itself.